The present application is generally directed to a method and apparatus for detecting alpha radiation in the presence of beta radiation and more particularly to a method and apparatus utilizing alpha spectroscopy for detecting alpha radiation in the presence of beta radiation.
Some separation operations require the detection of very low levels of alpha radiation in presence of very large amounts of beta radiation in a short time, preferably without generating hazardous wastes. An example includes the production of an isotope of molybdenum, Mo-99, for medical use. One decay product of Mo-99, technetium-99 (Tc-99), is used on patients in the U.S. thousands of times a day to conduct imaging of major organs for diagnosis of many conditions, including blockages and poorly functioning organs, thereby replacing invasive surgery as a means of detection. Because of its use within the human body, Mo-99 has to be very pure, requiring that the alpha contamination to Mo-99 activity ratio be less than 1.times.10.sup.-10.
Verification of the purity of Mo-99 presently comes through the chemical analysis of the solution containing the isotope. However, this method is time consuming, and produces an array of chemical wastes, which in turn have to be analyzed and disposed of properly, requiring additional time and resources.
Two basic approaches are presently utilized in the detection of alpha contamination in a sample. The first technique employs radiochemistry to separate and isolate a particular radioactive element. The sample is digested in an acidic solution and run through an ion-exchange column to separate the element of interest. The resulting solution is mounted on a filter and analyzed by an alpha spectrometer to identify and quantify the alpha-emitting radioisotopes. McKibbin (U.S. Pat. No. 5,190,881, issued on Mar. 2, 1993) describes a method of determining the radioactivity of uranium, plutonium, and americium in urine and fecal samples by this technique. Horwitz et al. (U.S. Pat. No. 4,835,107, issued on May 30, 1989) also describe a method and apparatus using this general technique for the quantitative recovery of actinide values from biological samples such as urine, blood and feces and from environmental samples such as soil and water. One significant problem with this technique is that it generates a significant amount of hazardous waste mixed with radioactive materials and is time-consuming and labor intensive.
The second method is used mostly for air filter samples where they are counted directly by a gas proportional detector. The properties of the detector are used to label the alpha and beta particles. Manipulating the high voltage on the detector that effects the pulse height of the incident radiation separates the alpha pulses from beta pulses. However, even under the ideal conditions, there is a small cross talk (mislabeling) from the beta into the alpha channel. In cases where the beta activity is very large, this produces a false positive signal for the alpha radiation. For activity levels above 1.times.10.sup.6 disintegrations per minute, the detector system becomes saturated (Kleinknecht, K., Detectors for Particle Radiation, 1998, Cambridge University Press, pp. 50-55).
Diamondis (U.S. Pat. No. 5,489,780, issued on Feb. 6, 1996) provides an example of a radon gas detector that employs a photovoltaic alpha particle detecting photodiode disposed within a radon gas detection chamber. Mohagheghi et al. (Mohagheghi, A., Ghanbari, F., Ebara, S., Enghauser, M. and Bakhtiar, S., J. of Radioanalytical and Nuclear Chemistry, 1998, Vol. 234, Nos. 1-2, 261-266) also describe a method of detecting alpha-emitting isotopes from air filters utilizing an alpha spectrometer. Mohagheghi et al. utilize a mathematical function to estimate the activity of each isotope and therefore support detection of the alpha particles. The method does not address detection of alpha particles in a beta field of radiation.
The problem of detecting low concentrations of alpha particles in a beta radiation field without generating additional hazardous wastes are solved by the apparatus and method discussed here. An alpha spectrometer is equipped with means for generating a magnetic field and a controller to control the ratio of beta and alpha particles that enter the detector of the alpha spectrometer. By proper control of the strength of the magnetic field applied around the alpha spectrometer, alpha particles can be detected from samples with alpha to beta particle activity ratios much less than 1.times.10.sup.-6.